Sun struck

By Jeff Hecht

EVERY 100,000 years or so, the Earth plunges into an ice age. Though it’s the planet’s strongest climate variation, no one has ever explained what causes this inexorable cycle. But the latest idea focuses on fluctuations in the Sun’s magnetic activity.

The traditional explanation for ice ages is that variations in the Earth’s orbit called Milankovitch cycles change how much energy reaches the planet’s surface from the Sun. Yet nagging doubts remain. In particular, the 100,000-year ice-age cycle doesn’t match either of the two strongest orbital variations—the wobbling of the Earth’s axis once every 23,000 years, …

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